This invention relates to low-pressure mercury vapor discharge lamps, and more particularly relates to such lamps which utilize amalgam to achieve a measure of temperature-independent operation.
Low-pressure mercury vapor discharge lamps, commonly referred to as fluorescent lamps, generally take the form of a sealed glass tube containing electrodes, mercury (Hg) and an inert gas, and internally coated with a phosphor layer.
The electrodes provide an electrical interface between external power and an internal plasma that forms between the electrodes during lamp operation. The mercury vapor absorbs electrical energy and emits UV radiation, while the phosphor layer absorbs UV radiation emitted by the mercury and emits longer wavelength UV radiation and/or visible light, depending on the particular type of lamp and its intended application.
The mercury vapor pressure is key to the efficient operation of such a fluorescent lamp. If too little mercury vapor exists in the plasma discharge, little UV radiation will be produced, and if too much mercury vapor exists in the plasma discharge, some of the UV radiation will get reabsorbed by the excess mercury vapor, reducing light output and causing inefficient power usage.
A common technique for control of the mercury vapor pressure in the discharge space is to design the lamp with a cold spot to which excess mercury vapor can condense, so that an equilibrium is established between the condensed mercury and the mercury vapor. Unfortunately, this technique is only useful over a relatively narrow temperature range. When the temperature in the discharge vessel increases much above this temperature range, for example, because the ambient temperature increases, the output efficiency decreases.
Another well-known method for control of the mercury vapor pressure involves the use of a mercury amalgam. This method allows the lamp to operate efficiently at a wider range of temperatures than standard fluorescent lamps employing cold spot technology. This method is commonly used in compact fluorescent lamps, but rarely used in linear or U-bent fluorescent lamps.
Using amalgams in fluorescent lamps requires methods which ensure against loss of mercury to the environment during handling and storage; which ensure that the amalgam stays in the proper location during lamp processing and operation; and which is compatible with high speed fluorescent lamp equipment and processes while also being cost effective. If sealed amalgam capsules are used, then the method must include the step of opening the capsule in the finished sealed lamp. See in this regard U.S. Pat. No. 4,288,715 and EP 0 063 393.
International Patent Application Publication WO 2006/000974 A2, the entire specification of which is incorporated by reference herein, discloses a low-pressure mercury vapor discharge lamp employing an amalgam to control mercury vapor pressure. The lamp is said to offer a compact design by locating the amalgam in a glass capsule (12) mounted inside the tubular glass vessel (2) which encloses the discharge space (3). The capsule (12) encloses an amount of amalgam (13) and has a gas opening at one end to enable gas exchange between the amalgam (13) and the discharge space (3). A glass rod (15) is located between the amalgam (13) and the gas opening (14) to prevent the plug of amalgam (13) from escaping from the capsule (12).
A drawback of the above arrangement is that the rod is not secured in the capsule and thus can move during lamp processing and/or operation, enabling the amalgam to move relative to its optimal location in the lamp, and even allowing portions of the amalgam to escape from the capsule.
According to the invention, a low-pressure mercury vapor discharge lamp comprises a glass vessel enclosing a discharge space containing mercury and an inert gas, the glass vessel being provided with at least one glass stem located within an end portion of the glass vessel, and also comprises a container encapsulating at least one plug of amalgam, the container being provided with an opening enabling gas exchange between the amalgam and the discharge space, the container also being provided with at least one spacer located between the amalgam and the opening to restrain movement of the amalgam within the container, the lamp being characterized in that the container also includes means for restraining the movement of the spacer within the container.
For the sake of convenience, the term ‘amalgam’ as used herein means both an amalgam of mercury with one or more other metals or alloys, as well as the pre-amalgam metal(s) or alloy(s) which may initially reside in the container or capsule, and which become amalgamated with mercury during subsequent lamp start-up and/or operation. Starting with a pre-amalgam metal or alloy in the capsule eliminates the potential loss of mercury during storage, handling and lamp manufacture. Mercury is then introduced into the lamp separately, in any of several known ways.
Suitable amalgam-forming metals and alloys are well-known, and include for example, indium, bismuth-indium alloys, and silver, lead, tin and their alloys.
The capsule is mounted inside the discharge space of the lamp, for instance, to an electrode shield or a metal collar easily attached (e.g., by welding) to an inner lead wire or an existing auxiliary wire of the lamp. The capsule, via its location and distance from the electrode, sets the operating temperature of the amalgam and constrains its movement at lamp operating temperatures.
Suitable means for restraining the spacer(s) in the capsule include metal wires and projections or annular ridges on the inner wall of the capsule. Suitable spacers include rod(s) and/or tube(s) of a heat-resistant glass, such as quartz or borosilicate glass.
According to a preferred embodiment of the invention, the capsule contains a main amalgam plug near the closed end of the capsule, an auxiliary amalgam plug near the opening in the capsule, a first spacer located between the main amalgam plug and the auxiliary amalgam plug, and a second spacer located between the auxiliary amalgam plug and the opening in the capsule.
According to another preferred embodiment of the invention, the amalgam capsule contains a magnetic plug in addition to the amalgam plug, and the amalgam capsule is moveably mounted inside a larger capsule having a fixed position within the discharge space, so that the amalgam capsule can be moved by means of an external magnet after lamp sealing has taken place, enabling adjustments to the location of the amalgam capsule to compensate for varying internal and/or external conditions.
The invention is usefully embodied, for example, in highly loaded substantially temperature-independent linear fluorescent lamps such as those having tubular envelopes of the type “T8”, efficiently operable over a temperature range of about 20 to 150 degrees F. at a power rating of about 80 to 85 W, compared to a temperature range of about 32 to 77 degrees F. and a power rating of about 25-40 W for standard T8 lamps without amalgam.
The invention is compatible with modern high speed horizontal lamp-making equipment, and can be readily integrated into existing manufacturing technologies for linear fluorescent lamps such as the T8 lamp.
The Figures are diagrammatic and not drawn to scale. The same reference numbers in different Figures refer to like parts.